Nightmare is an old, old, old shitty mutator I made that was designed to make Unreal enemies tougher. EXU Nightmare was the same idea, but designed to make Unreal enemies more like EXU-level of tough, but it was still shit. I wouldn't bother with it; it's a pretty poor implementation since it was written so long ago.

Lightning Hunter created Limbo as a spinoff, which was a lot better and more flexible (for Unreal enemies at least).